The invention relates to a thread-forming screw, and in particular a bone screw.
Thread-forming screws are widely known in the form of self-tapping or self-drilling screws. They have the advantage that no thread has to be precut. Such a screw is known, for example, from DE 20 2004 011 145 U1 and is used there as a masonry screw, for example as a brick screw. The screw described there has a preforming region with a pilot thread, adjoining which is a first thread section, then a threadless section and then at least one further thread section which is arranged below the screw thread.
Self-tapping and self-drilling screws are also used in other fields, for example in medical engineering, and here in particular in the area of osteosynthesis (for example in the skull area, in the oral and maxillofacial area, in the hand area or in trauma surgery). The screws used in this case are screws whose shanks are of cylindrical or conical design over the entire shank length (with the exception of the pointed or blunt screw end) and which are provided with a continuous thread of constant pitch. Aids which are indispensable in osteosynthesis, such as, for example, bone plates or plates of distractors, are fastened by means of these screws.
In applications in the skull area or especially in the oral and maxillofacial area, the diameters of such screws are of course as small as possible. However, since the screws have to be fixedly anchored in the bone, they often have a not inconsiderable shank since the screws have to be fixedly anchored in the bone, they often have a not inconsiderable shank length, even more so when a transmucosal or transgingival fastening is considered and it is thus desired to avoid a surgical preparation of the bone (by cutting open or lifting away mucosa or gingiva).
However, some difficulties may occur when screwing in such screws. For example, the torque increases quite considerably with increasing screw-in depth, which in particular in the case of longer screws requires the application of quite considerable forces by the surgeon, and that in a typically very restricted operating zone. This applies both to self-tapping screws, for which a core hole is produced beforehand in the bone in a separate step, and even more so to self-drilling screws, which are screwed without pilot holes into the bone, which saves a processing step (namely the pilot drilling) for the surgeon and can even lead to better anchoring of the screw in the bone.
In addition, when applying the requisite torques, which become higher and higher with increasing screw-in depth, the relatively thin screw shank may break (torsion fracture), such that the rest of the screw shank already in the bone has to be removed in a complicated manner in such a case. This can certainly be prohibited somewhat in the case of self-tapping screws by increasing the diameter of the core hole. To this end, however, either a further processing step is required (opening out the original core hole using a drill bit of larger diameter) or drilling is carried out immediately with a drill bit of larger diameter, there then being the risk of getting the drill bits mixed up and of not drilling with the correct drill bit. In addition, the anchoring of the screw in the bone in the case of a larger diameter of the core hole is of course slightly poorer.
Both the additional processing step of the further pilot drilling and the risk of mix-up when selecting the drill bit and also the application of partly very high forces for producing the requisite torque are disadvantageous for surgeons. The present invention comes in here, the object of which is to propose a screw which can provide a remedy in this respect.